1. Field of the invention
The present invention relates, in general, to a novel organometallic zirconium precursor and its in-situ synthesis and, more particularly, to an organometallic zirconium precursor with an improvement in volatility. Also, the present invention is concerned with a lead-zirconium-titanium thin film using the same and with a preparation method for preparing the same.
2. Description of the Prior Art
Lead-zirconium-titanium (PbZr.sub.x Ti.sub.1-x O.sub.3) thin films or its related thin films (hereinafter referred to as "PZT thin films"), used as ferroelectric thin films for semiconductor capacitors, are prepared from organometallic precursor sources, e.g. lead source, zirconium source and titanium source. For production of the PZT thin films, there is generally used a metal-organic chemical vapor deposition process (hereinafter referred to as "MOCVD"), a technique for growing thin layers of compound semiconductors. In practice, MOCVD comprises preheating a liquid or solid of organometallic precursor to vaporize it and decomposing the vapor by heat or plasma, to deposit a thin film.
It is necessarily required that appropriate organometallic precursors be selected in order to obtain desired properties and quality of the PZT thin film. The following are the general conditions that organometallic precursors should have: first, organometaliic precursors should be able to be easily volatilized at low preheat temperatures, in addition to being stable at the preheat temperatures; second, the difference between the vaporization temperature and the decomposition temperature should be large enough; third, it is preferred that the organometallic precursors are not decomposed or changed by the moisture contained in air; finally, they should not be environmentally toxic.
Of the various organometallic precursors useful for preparing the PZT thin films, organometallic Ti precursors and organometallic Pb precursors are relatively well developed and may be volatilized at relatively low temperatures. By contrast, organometallic Zr precursors are disadvantageous in that they are very hygroscopic or difficult to volatilize.
Conventionally, it is tetra (2,2',6,6'-tetramethyl-3,5-heptanedione) zirconium (hereinafter referred to as "Zr(THD).sub.4 ") that is the most useful in providing a zirconium source for PZT thin films. Zr(THD).sub.4 exhibits outstanding thermal stability that it is not decomposed at up to 300.degree. C. In addition, Zr(THD).sub.4 is very resistant to moisture, unlike other Zr precursors. However, this Zr source for PZT thin film has a vaporization temperature that is much higher than those of the other sources, Pb and Ti sources. In other words, the gap in vaporizing point between the Zr source and the other sources is large. For example, Zr(THD).sub.4 is volatilized at a temperature of about 220.degree. to 240.degree. C. whereas Pb(THD).sub.2 and Ti(OCH.sub.2 CH.sub.3).sub.4 (hereinafter referred to as "Ti(OEt).sub.4 ") have a vaporizing point of about 140.degree. to 160.degree. C. and about 110.degree. to 120.degree. C., respectively.
Accordingly, it is difficult to control the temperature of a premixing chamber or preheat tube in which volatilized Pb, Ti and Zr precursors are mixed just before entering a reactor. For example, if the preheat temperature is controlled for the Pb or Ti precursor, the gas phase Zr(THD).sub.4 is highly apt to be solidified. On the other hand, if the preheat temperature is raised for the Zr precursor, the Pb and Ti precursors are decomposed or changed before entering the reactor. Further, preheating to the high temperature of around 250.degree. C. lays great restraint on selection of materials for bubbler or valve sealing. Consequently, in spite of superior thermal stability and moisture resistance, Zr(THD).sub.4 is disadvantageous because of its low volatility.